Jacquelinne J. Acuña

Indole acetic acid and phytase activity produced by rhizosphere bacilli as affected by pH and metals

The abilities to produce indole acetic acid (IAA) and mineralize organic phospho- rus by phytase are desirable traits in plant−growth promotion rhizobacteria (PGPR) particularly in Chilean Andisols which are characterized by low pH and high total P. However, little is known about the influence of soil properties that are specific to An- disol (low pH and metal toxicity) on the effectiveness of PGPR. Here, we assessed the effect of pH and metal cations on IAA and phytase activity of cell-associated proteins produced by two bacilli strains isolated from the rhizosphere of pasture plants. The production in vivo of IAA by Paenibacillus sp. SPT−03 was significantly increased (7−fold) when incubated in tenfold diluted culture medium, compared to the full-strength medium. At low pH (pH<5), phytase activity of cell−associated proteins and IAA production of Bacillus sp. MQH−19 was decreased, whereas they were increased in Paenibacillus sp. SPT−03. Moreover, phytase activity in vitro of cell−associated proteins and IAA production in both bacilli strains were signifi- cantly inhibited by 30−100% and 44−70% by concentrations of 10 mM and 350 µM Fe 3+ and Al 3+ , respectively. At 350 µM Mn 2+ IAA production was inhibited by 30−100% in both strains but there was no effect on phytase activity. This study shows that certain properties of Andisol may differentially affect some mechanisms related with PGPR efficiency.

Effect of phosphorus addition on total and alkaline phosphomonoesterase-harboring bacterial populations in ryegrass rhizosphere microsites

Rhizobacterial communities may play a crucial role in phosphorus (P) nutrition of plants. However, our knowledge of how P fertilization modulates rhizobacterial communities in crops and pastures is still poor. Here, we investigated the effect of P addition (phosphate [PHO] and phytate [PHY]) on the composition of total bacterial communities and alkaline phosphomonoesterases (APase)-harboring bacterial populations in the rhizosphere microsites (root tip [RT] and mature zone [MZ]) of L. perenne. Sizes and diversities of bacterial communities were studied by 454-pyrosequencing of 16S rRNA genes, denaturing gradient gel electrophoresis (DGGE) and quantitative PCR (qPCR). Our results suggested that phosphorus addition induces significant changes in the rhizobacterial community composition. Despite that pyrosequence analysis showed that members of the Proteobacteria, Actinobacteria, Chloroflexi, and Acidobacteria were the dominant phyla in all sampled rhizosphere microsites, differences in the relative abundances of some bacterial genera were detected (e.g., Arthrobacter and Acidothermus). Greater richness in rhizosphere microsites of plants supplied with PHY compared with PHO were revealed. With respect to APase-harboring bacterial populations, DGGE (phoD gene) showed significant differences between microsites supplied with PHO, PHY and controls. qPCR (16S rRNA genes, phoD and phoX) showed significantly greater abundances of bacteria and APase genes in RT than in MZ microsites. This study contributes to our understanding of the effect P fertilization on rhizobacterial community compositions of pastures grown in Chilean Andisols.

Bacterial alkaline phosphomonoesterase in the rhizospheres of plants grown in Chilean extreme environments

Bacterial alkaline phosphomonoesterases (APases) are relevant for organic phosphorus (Po) recycling in many soils. However, the abundance and diversity of bacterial APase in the rhizospheres of native plants are poorly known, particularly in extreme environments. In this research work, we studied the composition of total and APase-harboring bacterial communities, abundances of selected APase genes (phoD and phoX), and APase activities in rhizosphere soils from native plants grown in extreme environments of northern (Atacama Desert), central (Andes volcano; Quetrupillan and Mamuil Malal) and hot spring (Liquiñe), and southern polar (Patagonia and Antarctic) regions of Chile. Differences in the composition of bacterial communities in the rhizosphere soils were revealed by denaturing gradient gel electrophoresis (DGGE) and quantitative PCR (qPCR) of 16S ribosomal RNA (rRNA), phoD, and phoX genes. In general, the significant lowest bacterial diversities, APase gene abundances, and APase activities were observed in rhizosphere soils from Atacama Desert, whereas the highest values were observed in rhizosphere soils of Patagonia. In addition, APase gene abundances were positively correlated among them and with APase activity of rhizosphere soils, but negatively correlated with phosphorus (P) availability in rhizosphere soils. Although bacterial APases were observed in all studied rhizosphere soils, their relevance to soil Po recycling in soils of extreme environments remains unclear and further studies are needed.

Rhizobacterial Community Structures Associated with Native Plants Grown in Chilean Extreme Environments

Chile is topographically and climatically diverse, with a wide array of diverse undisturbed ecosystems that include native plants that are highly adapted to local conditions. However, our understanding of the diversity, activity, and role of rhizobacteria associated with natural vegetation in undisturbed Chilean extreme ecosystems is very poor. In the present study, the combination of denaturing gradient gel electrophoresis and 454-pyrosequencing approaches was used to describe the rhizobacterial community structures of native plants grown in three representative Chilean extreme environments: Atacama Desert (ATA), Andes Mountains (AND), and Antarctic (ANT). Both molecular approaches revealed the presence of Proteobacteria, Bacteroidetes, and Actinobacteria as the dominant phyla in the rhizospheres of native plants. Lower numbers of operational taxonomic units (OTUs) were observed in rhizosphere soils from ATA compared with AND and ANT. Both approaches also showed differences in rhizobacterial community structures between extreme environments and between plant species. The differences among plant species grown in the same environment were attributed to the higher relative abundance of classes Gammaproteobacteria and Alphaproteobacteria. However, further studies are needed to determine which environmental factors regulate the structures of rhizobacterial communities, and how (or if) specific bacterial groups may contribute to the growth and survival of native plants in each Chilean extreme environments.

Improving selenium status in plant nutrition and quality

Abstract Selenium (Se) is an essential micronutrient for human health due to its antioxidant capabilities. The Se content around the world is highly variable from 0.005 mg kg -1 in areas from China and Finland to 8,000 mg kg in seleniferous soils from Tuva-Russia. However, about one billion of people in the world wide are Se deficient. During the last decade, studies related with strategies for Se biofortification in food plants for human nutrition have significantly increased because this metalloid is incorporated into human metabolism mainly as a constituent of food plants. Similarly, Se biofortification is important in pastures for increasing the Se content in cattle to enrich meat and to prevent disease associated to Se deficiency as white muscle disease. In China, two endemic diseases have been related to Se deficiency: Keshan and Keshin–Beck diseases.Agronomic biofortification by using inorganic Se sources is a current practice in countries as China, Finland, and USA. In Chile, fertilization by using chemical compounds with Se is an uncommon practice due the edaphoclimatic characteristics of Andisols, which represent around 60% of agricultural soils of southern Chile. Recent studies showed that microorganisms as bacteria and arbuscular mycorrhizal fungi play an important role in the transformations and Se availability, representing an interesting biotechnological alternative to Se biofortification.This review is focalized to describing Se behavior in soil-plant system and the possible strategies to improving Se content, including the use of microorganisms as biotechnological tools for increasing plant nutrition and quality. Specific attention will be devoted to volcanic soils of Southern Chile, where different factors concur to enhance the Se-deficiency problem.

Inoculation with selenobacteria and arbuscular mycorrhizal fungi to enhance selenium content in lettuce plants and improve tolerance against drought stress

This study evaluated the co-inoculation effect of the endophytic selenobacteria Bacillus sp., Klebsiella sp. or Acinetobacter sp. and the arbuscular mycorrhizal (AM) fungus Rhizophagus intraradices on lettuce plants grown under drought conditions. Plants inoculated with bothnmicroorganisms were able to enhance the Se content in their shoots (1 to 6 pg plant-1) and promote macro-and micronutrient uptake. Moreover, the inoculated plants showed significantntolerance to drought stress, as determined by their adaptation to physiological parameters(relative water content and stomatal conductance), increase in photosynthetic pigments (chlorophyll and carotenoids) and improvement inantioxidant enzyme levels (catalase, ascorbate peroxidase and glutathione reductase). The selenobacteria increased the Se content in lettuce plants and enhanced the effect of AM fungus in controlling the antioxidant systems that play a role as elicitors of plant drought responses and improving the nutritional quality and physiological and biochemical processes involved in plant drought tolerance.

Bacterial Community Profile of the Gut Microbiota Differs between Hypercholesterolemic Subjects and Controls

The role of gut microbiota in the development of metabolic illnesses has been abundantly demonstrated. Recent studies suggest that gut microbiota alterations may also be related to the development of hypercholesterolemia. Therefore, we aimed to assess differences in the gut bacterial community profiles between hypercholesterolemic subjects and controls. Thirty cases diagnosed with hypercholesterolemia and 27 normocholesterolemic controls were included. A fasting whole blood sample was obtained to determine the lipid profile. In parallel, stool samples were collected and total DNA was isolated to assess the bacterial community profiles by denaturing gradient gel electrophoresis (DGGE). In addition, the Richness, Shannon-Weaver, and Simpson indexes were used to evaluate the richness and diversity of bacterial communities. As expected, serum concentrations of total cholesterol, triglycerides, and LDL-cholesterol were significantly higher in the cases compared with controls. Moreover, DGGE analysis showed a lower richness and diversity of bacterial communities in hypercholesterolemic subjects. In conclusion, our results showed differences in the profiles of bacterial communities between hypercholesterolemic subjects and controls, suggesting a possible role of the gut microbiota in the development of hypercholesterolemia.